1. Field
The present invention relates generally to wireless communication, and more specifically to cellular wireless communication.
2. Background
The field of communications has many applications including, e.g., paging, wireless local loops, Internet telephony, and satellite communication systems. An exemplary application is a cellular telephone system for mobile subscribers. (As used herein, the term “cellular” system encompasses both cellular and personal communications services (PCS) system frequencies.) Modern communication systems, such as a wireless communication system, designed to allow multiple users to access a common communications medium have been developed for such cellular systems. These modern communication systems may be based on multiple access techniques such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), space division multiple access (SDMA), polarization division multiple access (PDMA), or other modulation techniques known in the art. These modulation techniques demodulate signals received from multiple users of a communication system, enabling an increase in the capacity of the communication system. In connection therewith, various wireless communication systems have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile communication (GSM), and other wireless systems.
In FDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands and each user is given its own sub-band to access the communication medium. Alternatively, in TDMA systems, the total frequency spectrum is divided into a number of smaller sub-bands, each sub-band is shared among a number of users, and each user is allowed to transmit in predetermined time slots using that sub-band. A CDMA system provides potential advantages over other types of systems, including increased system capacity. In CDMA systems, each user is given the entire frequency spectrum for all of the time, but distinguishes its transmission through the use of a unique code.
Frequency hopping is a technique employed in a number of communication systems to provide frequency diversity over time. A frequency hopping system transmits on a different (usually narrowband) carrier (also referred to herein as “carrier frequency”) during every transmission unit or slot. The sequence of the carriers that are employed (hopping sequence) is usually such that hops between non-contiguous frequencies are performed, in order to better combat frequency-selective fading. An example of a system employing frequency hopping is GSM/GPRS/EDGE (also referred to as GERAN).
Channel quality estimation, which includes signal strength measurements, is one of the key characteristics of those systems employing a fast feedback scheme, which then exploits multi-user diversity. In such systems, the receivers estimate the quality of their reception from a base station according to some predefined metric (e.g. C/I strength). This is generally performed after a beacon or pilot is measured. The beacon or pilot can be non-intermittently (CDM or Code Division Multiplex) or intermittently (TDM or Time Division Multiplex) transmitted by the base station. The receivers report the measured quality to the base station with appropriate messages or indications. The base station can then exploit this information to schedule subsequent transmissions, in a fashion that is depending on the details of the scheduling algorithm employed by the network itself. In particular, the network can use this information to schedule a transmission towards the user experiencing the best channel conditions (multi-user diversity), or adapting the modulation and coding to the channel conditions of the selected user (fast link adaptation).
The mechanism described above generally relies on the fact that the system employs the same carrier. In other words, the measurement process performed by a receiver takes place on the same frequency that is used for subsequent scheduled transmissions. This is one of the requirements for the scheduling process to be able to take into account the received reports. This requirement cannot be fulfilled in most frequency hopped systems. In such systems, beacon measurements and scheduled transmissions would often be on separate, uncorrelated carriers. Even if the channel is correlated (hopping within the channel coherence bandwidth), the interference could be drastically different from one carrier to another. Therefore, the reporting-scheduling process would not be able to exploit the selectivity of fading/interference thus providing the corresponding gain. Such a limitation is illustrated in FIG. 8 in which a remote station's receiver receives data 800 on carrier 1 in frame 2 and then hops to carrier 2 in frame 2 to receive data 802.